Generation of subpicosecond electrical pulses by nonuniform illumination of GaAs transmission-line gaps

We present a detailed study of subpicosecond pulse generation by nonuniform illumination of transmission-line gaps on semiinsulating GaAs. The dependence of such pulses on bias voltage, light intensity, and wavelength was examined in detail with the aid of a subpicosecond electrooptic sampling system. A complete spatial mapping of the excitation area indicates that the pulse generation is due to the depletion of the electrical field in the illuminated section of the gap. A comparison of uniform and nonuniform gap excitation schemes pinpointed the physical differences between the two processes of electrical-transient generation. Picosecond pulses were also generated by nonuniform illumination of a photoconductive gap placed in series with a coplanar waveguide for the first time and were found to contain balanced, odd modes only.>